I understand an iso traf connects two separate transformers magnetically - one isolated from the mains. I have a medical-grade iso traf where the label says the secondary's neutral is not grounded. Other than noise-free AC required by medical equipment, what's the safety benefit of an iso-traf for repair projects such as the case of a 2-prong plugged classic radio?

I understand an iso-traf does not provide 100% protection against shock but besides personal safety what's the benefit in terms of protecting test equipment & equipment under repair?

well, generally speaking, a good and carefully designed isolation transformer and labeled as such (isolation that is) will not only provide protection against the live line of the AC power but also suppress common mode noise and ground loop noise as well as block very high voltages from reaching the secondary before they die :-) . So if you are careless enough to touch either of the secondary terminals (but not both ) while being barefoot on ground or other large conducting surface you will not suffer electrocution - probably ;-)

a good and carefully designed isolation transformer and labeled as such (isolation that is) will not only provide protection against the live line of the AC power but also suppress common mode noise and ground loop noise as well...

I plugged my integrated amp (Naim Nait-1) into the Toroid iso-traf, the noise drop was significant. The difference was at least as good as adding a separate Naim PSU which sells used/new between $700-1500. Nait-1 is not upgradable by a separate Naim PSU but more modern Naim pre-amps and cd players are. With Nait-1, it's possible to upgrade the internal monolithic power regulation circuit with a super-regulator. Naim recommends against noise-supression, filters, etc claiming it will degrade or change the sound. Well with Toroid iso-traf plugged in, Naim's unmistakable PRAT is still there and grooving better than ever. To my ears it sounds as good if not better as adding one of their Hi-Caps (external PSU) - maybe it's the psycho-acoustic effect of paying less and getting something in return . For me personally, it invalidates the marketing pitch they've been pushing for decades.

In the US a isolation transformer is normally wired as a Separately Derived System (SDS) which is covered in the NEC code book starting at about Article 250.28. Medical systems are covered in Article 517 Health Care Facilities.

That is a great way, speedskater. Unfortunately, there is a vocal and uninformed population on this forum (but not this thread, not yet) that will continue to insist on the benefits and usefulness of floating power transformer secondaries. They will claim safety, then claim "don't care" when you reference the NEC, and how it does not permit floating systems. NEC was designed with safety in mind.

Medical equipment, sure. But our audio systems are not used to perform surgery, nor do we install Line Isolation Monitors. Balanced power, per NEC 647, sure, but not in the residence; read carefully.

Bond the secondary. The old argument that if you accidentally contact the hot, a floating system will not hurt you misses the point of safety requirements, making the case that two wrongs make a right.

It is still an isolation transformer even when the secondary is bonded. It is still a separately derived system when the secondary is bonded. It is not a floating system when the secondary is bonded, and you don't want that.

Hmmm.... I'm no expert here but I think a different conversation is called for. RIZE had this question:

Quote:

Originally Posted by rize

..... what's the safety benefit of an iso-traf for repair projects such as the case of a 2-prong plugged classic radio?

So what we're looking for here is how to put in place a "tech bench" isolation transformer, and then to understand what safety benefit it provides.

I did a lot of online searching on this topic last February. Now, there's a lot of argument, pro's and con's to various set ups, but I'm assuming (1) you're looking to repair AC-powered devices, and (2) you may be using an oscilloscope to check circuits, (3) you understand that there's no device that is going to make you safe if you do stupid things, and (4) unless you understand circuits, it's easy to do stupid things.

That said, many people recommend a "tech bench" isolation transformer -- defined as one where the trafo secondary provides power AND has no connection to earth ground.

Why is this "safer" ? The argument is that most electric shocks do not occur because someone simultaneously touches two supply wires, rather they more frequently occur when someone touches a single live wire and a "ground". The isolation transformer has a secondary which is non-grounded, thus you can avoid being shocked by touching just one wire and "ground".

Here's the other information I found online that seem worth listing:

1. The best way to avoid getting shocked is using safe handling procedures. It means to always make sure your body is not going to become the conduit for electricity to move -- high voltage to low, any voltage to ground. One rule is to only have one hand probing the device under test-- never both.

2. An isolation transformer only removes some of the hazard of working on electrical equipment, i.e, the risk that you'll become a path from voltage to ground. The downside is that this may lead to carelessness. Go back to rule 1. Constant use of safe handling procedures are the only insurance against electric shock, and the use of an isolation transformer won’t fundamentally change that.

3. There are various isolation transformers available, but I would not trust that the secondary is isolated (disconnected) from ground unless I disconnected it myself, or tested it to make sure. According to some sources, “publicly available” trafos, including the hospital isolation transformer has to be modified for a tech bench... the public/hospital equipment connects the secondary to the [house] ground wire. (See for example TomFun.com for discussion).

4. Ideally, include a surge supressor to the set up, as follows:Outlet-àsurge supressoràisolation transfomeràdevice you are testing.

5. Compare the isolation transformer's rating to the power demands of the equipment you want to test, repair or operate. Make sure the transformer's power and current ratings are higher than those of the equipment.

So ..... From what I make of it, the "tech bench" isolation transformer is particularly useful if (1) the device under test doesn't have its own "mains isolating" (internal) transformer, or (2) is a two-wire, "ungrounded" device.

If the device you're testing has a three-prong plug, realize that you can still use your isolation transformer, but ... you've just disabled it's safety-fuse-to-ground circuit. That fuse isn't going to blow! A short to the chassis may make the chassis "hot" and now you don't have that "safety" in place, as protection to you or the device components. !! Maybe not so good. Something to think about.

I hope that helps. I don't feel that this answer is complete, ---- I'm hoping others will join in. I'd like to understand this topic better !

Last edited by truepaul; 20th October 2012 at 02:49 AM.
Reason: another typo

Hmmm... I just re-read the previous post, and it looks like I may be illustrating zigzagflux's argument ... i.e., advocating the "benefits and usefulness of floating power transformer secondaries." Isn't there a place for this when we're talking test equipment, not audio systems themselves?